Conventionally well-known thermal recording materials use a coloring reaction of a dye precursor and a color developer that develops the color of the dye precursor upon contact with the dye precursor under heating, both coloring substances being melted and brought into contact with each other by heating, thereby obtaining colored images. Such thermal recording materials are relatively inexpensive, and require compact recording devices and easily maintenance of recording devices; therefore, they are used in a wide range of fields as recording media for facsimiles, word processors, various calculators, and other applications.
In accordance with the expansion of their applications, thermal recording materials are required to have various qualities, such as higher sensitivity, improved image stabilization, and multicolor recording capability.
Means of multicolor recording are advantageous in that, for example, letters and patterns to be emphasized can be markedly and clearly displayed in color different from other parts. In particular, multicolor thermal recording materials capable of recording in two or more colors from among red, blue, yellow, and black have excellent versatility, and their practical use is thus highly anticipated.
Attempts have been made to provide multicolor thermal recording materials that utilize the difference in heating temperature or heat energy, and various multicolor thermal recording materials have been proposed. Multicolor thermal recording materials generally comprise a high-temperature color-developing layer and a low-temperature color-developing layer that are sequentially laminated on a support and develop different colors. Such multicolor thermal recording materials are broadly classified into two types: decoloring materials and color-adding materials.
For example, PTL 1 to PTL 3 propose decoloring multicolor thermal recording materials in which a color-developing operation at a low temperature only develops the color of a low-temperature color-developing layer, and when a color-developing operation is performed at a high temperature, a decolorizing agent having a decoloring effect acts on the color-developing system of the low-temperature color-developing layer, and only the color of the high-temperature color-developing layer is obtained.
PTL 4 to PTL 6 propose color-adding multicolor thermal recording materials in which two thermal color-developing layers that develop different colors are laminated, and different amounts of heat are applied to thereby obtain two identifiable colors. Further, PTL 7 proposes a color-adding type multicolor thermal recording material in which two or more dye precursors developing different colors and having different average particle diameters are mixed in the same layer.
Moreover, PTL 8 proposes developing multiple colors by dissolving coloring components that develop mutually different colors in solvents, and encapsulating the resulting mixtures separately in two or more microcapsules having different glass transition temperatures.
In contrast, PTL 9 and PTL 10 propose multicolor thermal recording materials in which a dye precursor is formed into microcapsules or composite fine particles to thereby reduce its color-developing sensitivity, which is distinguished from the color-developing sensitivity of a dye precursor present in the form of solid fine particles, based on the difference in color-developing sensitivity.
However, such multicolor thermal recording materials had one or two color-developing layers, and it was possible to obtain only colored recording images in at most three colors (e.g., red, blue, and purple obtained by mixing red and blue).
In order to solve this problem, PTL 11 proposes a method for developing multiple colors by providing color-developing layers with three or more colors.
In the method of PTL 11, it is necessary to provide an intermediate layer between the thermal color-developing layers in order to control the temperature transmitted to each thermal color-developing layer. The formation of an intermediate layer between thermal color-developing layers causes problems of the increase in the number of times of coating during the production, and the reduction of the yield of each layer, consequently resulting in a significant cost increase. Moreover, dye precursors having different melting points are used in each color-developing layer in order to develop the color of each color-developing layer at a desired temperature; however, due to the restriction on the molecular structure of dye precursors, only limited dye precursors can be used, which results in problems in the selectivity of materials.